1. Field of the Invention
This invention relates to a head positioner employing a dual-stage actuator method. In the dual-stage actuator method, a record/replay head (“record” and “replay” hereinbelow will also be referred to as “write” and “read”, respectively) is position-controlled through cooperative operations of a coarse actuator and a fine actuator. This invention relates to a head positioner having a configuration employing the dual-stage actuator method, wherein a fine actuator is configured using a piezoelectric device.
Preferably, although the head positioner of the invention is primarily mounted on a magnetic disk drive, the invention is not limited thereby. The head positioner of the invention may be mounted another information storage apparatus, such as a magnetooptical disk drive, or an optical disk drive.
In addition, the invention also relates to an information recording/replaying apparatus represented by a magnetic disk drive.
2. Description of the Related Art
Conventionally, there are apparatuses for performing high-speed and high-accuracy positioning of a write/read head over an information storage disk such as a hard disk. These apparatuses include a dual-stage-type head positioner formed of a coarse actuator and a fine actuator(s). The fine actuator is generally called a “microacuator (MA)”, “dual-stage actuator”, or “piggyback actuator”. A piezoelectric device (PZT device) is widely used to form the fine actuator.
For the coarse actuator, voice coil motor (VCM) is generally used. The coarse actuator is used for large movement as in a seeking operation or a multitrack jumping operation. The coarse actuator drive a head-mounted supporting mechanism to rotationally move on an axis on a chassis. In contrast, the fine actuator used to perform high-speed and fine positioning as in a track following operation and a single-track jumping operation. The fine actuator is configured with the piezoelectric device being disposed between the coarse actuator and a head slider. The amount of head displacement is adjusted by controlling application voltage for the piezoelectric device constituting the fine actuator. The displacement stress increases proportionally to the increase in the level of the application voltage. This enables head positioning with improved sensitivity.
Nevertheless, however, using the piezoelectric device as the fine actuator arises problems described hereunder. With a high voltage being applied to the two electrodes of the piezoelectric device, a metal component in the piezoelectric device causes an electrochemical reaction, thereby precipitating the metal component in the electrodes. For example, a thin-film PZT device made of a lead-zirconia-titanium alloy. Practically, the PZT device is not formed only of PZT crystal, and the device is doped with, for example, lead oxide and water in the fabrication. When lead, water, and the like are included, the electrochemical reaction is activated in a voltage-applied state, phenomena occur in which lead is precipitated on the cathodic side. In proportion to the increase in the application voltage, the lead precipitation phenomena increasingly tend to occur. Although adverse effects of the lead precipitation do not immediately occur prominent, the lead precipitation gradually advances. When erosion phenomena due to long-term lead precipitation continue, the displacement property of the piezoelectric device deteriorates. Further continuation of the erosion phenomena can disable the piezoelectric device to be displaced. Finally, the lead precipitation causes shortcircuiting, thereby causing breakdown of the piezoelectric device.
As described above, while application of high voltage to the piezoelectric device satisfies requirements for the high-speed sensitivity, adverse effects due to long-term lead precipitation causes a problem in the service life of the device. Conventionally, sufficient study has not made regarding the balanced relationship among the positioning accuracy, high-speed sensitivity, and application voltage. As such, the problems as described above occur. The importance of resolving the problems increases in proportion to the increase to be made in the control band for satisfying the high-speed sensitivity.
Conventionally, there is a configuration employing a positioning method developed to implement an increased information mass-storage capacity. In the configuration, a plurality of information storage disks are coaxially disposed, fine actuators and heads are disposed corresponding to the disks, and a plurality of head supporting mechanisms individually mounted to the fine actuators are moved in batches by using a coarse actuator. Specifically, the aforementioned configuration includes a plurality of heads for independently accessing a plurality of disks, a plurality of fine actuators for finely displacing the individual heads, a plurality of head supporting mechanisms for individually supporting the fine actuators, and a coarse actuator for batch-driving the plurality of head supporting mechanisms. Generally known conventional methods for driving the coarse actuator include a method as described in the following paragraphs.
Weightings are individually added to relative displacement signals of a plurality of fine actuators, and a coarse actuator is controlled to converge each of the weighting-added signals to zero. The relative displacement signals are weighted according to importances preliminarily allocated to individual heads. The importance typically determines the priority in the access order. However, when the coarse actuator is controlled on the basis of the importances set to weighting points, and weightings are added to all the relative displacement signals, problems are arisen in that the head supporting mechanism is frequently moved, and oscillations are caused thereby.
When a large wobbly movement occurs with a desired track corresponding to a head having the highest importance, the highest-importance head is maximally displaced. Accordingly, the value of the corresponding relative displacement signal becomes largest, and the amount of operation of the coarse actuator is large in proportion to the greatest weighting added to the relative displacement signal of the highest-importance head. Thereby, the individual fine actuators are over-displaced. Since the frequency of access-command issuance to the highest-importance head is high, the individual fine actuators are induced to cause oscillations. The oscillations influence the structural stability of each of the fine actuators. In addition, structural resonances are caused due to inter-fine actuator mutual interference. A condition occurs such that the positioning control itself of the head toward a desired track introduces a positional deviation from the desired track. Consequently, for example, read or write errors tend to occur at the desired track. These problems will be described in more detail with reference to FIGS. 46 and 47.
As countermeasures to prevent occurrence of the oscillations, there is proposed an idea of increasing the speed of following operations to a desired track. In order to improve the sensitivity, however, a proportionally high voltage needs to be applied to the piezoelectric device, which constitutes the fine actuator. As described above, application of overvoltage causes the problems of lead precipitation, property deterioration, and in addition, breakdown of the piezoelectric device.